Most standards for allowable limits of airborne pollutants assume the exposures are to single agents, but in practice this rarely occurs and populations are exposed to mixtures containing several or many components. However, data and practical experience are accumulating which indicate that the biological effects of one component may well be influenced by the presence of other components. The purpose of this investigation is to quantitate the effects of one pollutant upon the effects of another, using a system of a dust and a gas each of which has the same principal biologic effect. To achieve this, guinea pigs will be exposed to fluorspar for several months and the effects quantitated in terms of fluoride concentrations in selected body fluids and tissues, including the respiratory tract, and by histological examination of sections of the lungs. Other guinea pigs will be exposed in similar fashion to hydrogen fluoride and these same effects again noted. Thirdly, guinea pigs will be be exposed to the two agents in combination. Results of the three experiments should indicate whether or not effects of the agents in combination are less severe (unlikely), the same, or more severe, and permit suggestions regarding mechanisms of biological interactions of pollutants. From a practical viewpoint, the results might help decide whether or not threshold limit values for single agents should be changed when these are present in combination.